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Money may not grow on trees, but chocolate does, Simon Cotton writes.
Money may not buy happiness or grow on trees, but when it comes to chocolate, it seems you can have both.
Chocolate really does grow on trees and the chemical feel-good factor comes from the world's most widely consumed psychoactive drug.
Theobroma cacao is an evergreen that is native to tropical regions of the American continent and its seeds or beans are the source of the 4 million metric tonnes of chocolate produced each year, much of it from countries such as the Ivory Coast and Indonesia.
Chocolate consumption goes back at least 4000 years, to the peoples of present day Mexico: the Mayans, Aztecs and their predecessors, the Olmec.
Just as today, they roasted the fermented seeds from cocoa pods, grinding the roast to a powder which they used to make a chocolate beverage, a cold, foaming drink that was very different from the substance we consume today.
Sometimes they added honey to sweeten it and the Aztecs also added chili pepper to give the phrase ''hot chocolate'' a whole new meaning.
Two thousand years ago, the Mayan people of what is now known as Guatemala even came up with the original ''chocolate teapot'', a ceramic vessel used to pour the foaming drink, and archaeologists have found evidence that chocolate drinks were served at the celebrations after the interment of sacrificial victims (though I'm not sure that the condemned would have been made any happier with a bar of chocolate).
The last Aztec emperor, Montezuma II, consumed a lot of this drink every day, and it was hinted that this enhanced his virility.
No wonder the Spaniards were interested. Of course, it was the Spaniards who brought this wonder drink back to Europe, but adding sugar and spices such as cinnamon and vanilla, another import from the Americas, transformed it into the much sweeter drink we have now.
Chocolate drinking became the thing to do in fashionable society.
Less than 200 years ago, the invention of the chocolate press by Casparus van Houten senior made it possible to separate roasted cocoa beans into cocoa butter and a solid that could be made into cocoa powder.
This powder could be recombined with sugar and cocoa butter to produce an eating chocolate, and in 1847 the Bristol Quaker firm of Fry's, closely followed by Cadbury's in Birmingham, made the first chocolate bar.
The Swiss came up with milk chocolate bars in the 1870s, and to this day Switzerland and Britain are two of the top nations for chocolate consumption. Chocolate Easter eggs were invented in the 1870s, and we haven't looked back since.
The taste of chocolate comes from a mixture of chemicals, many resulting from the roasting process, in which sugars and amino acids combine, forming members of a family of molecules called pyrazines, which contribute the nutty, roasted and chocolately sensations.
But what about the ''feel-good'' side of chocolate? For a start, there is the world's most widely consumed psychoactive drug: 1,3,7-trimethylxanthine by name.
You may have heard of it: we call it caffeine. It works by counteracting the natural neurotransmitter adenosine, resulting in an increase in heart-rate and muscle contraction.
There is also a significant presence of theobromine in chocolate, a similar stimulant which also happens to be the molecule that makes chocolate poisonous to dogs.
Then there is serotonin, a natural neurotransmitter which controls many functions in the brain, including mood and behaviour.
The body makes it from the natural amino acid tryptophan and chocolate contains both serotonin and tryptophan.
Another chocolate molecule believed to be important was discovered less than 20 years ago: anandamide. This binds to receptors in the brain known as cannabinoid receptors.
These receptors were originally found to be sensitive to the most important psychoactive molecule in cannabis, delta 9-THC. Likewise, anandamide and similar molecules found in chocolate are also thought to affect mood.
Phenylethylamine, another family of chemicals, is found in chocolate in very small amounts. It is a naturally occurring substance with a structure that is closely related to synthetic amphetamines, which, of course, are also stimulants.
It is often said that our brain produces phenylethylamine when we fall in love, and it acts by producing endorphins, the brain's natural ''feel-good'' molecules.
The bad news, however, is that eating chocolate is probably not the best way of getting our hands on phenylethylamine as enzymes in our liver degrade it before it can reach the brain.
There are yet more other molecules in chocolate - especially in dark chocolate - such as flavonoids, which some scientists think may help improve cardiovascular health (but chocolate manufacturers have been known to remove bitter flavanols from dark chocolate).
There is one feel-good factor I have not mentioned, which isn't a molecule - the melt-in-your mouth sensation.
The fatty triglycerides in cocoa butter can stack together in six different ways, each resulting in a different melting point.
Only one of these forms has the right melting point of about 34degC, so that it ''melts in your mouth, not in your hand''.
Getting the chocolate to crystallise to give this form is a very skilful process, the product of very careful chocolate engineering.
There is still much yet to know about chocolate and some are now even sequencing the genome of cultivated cacao.
But the continuing intricacies in chocolate and cacao that we are discovering through science can only add to the very simple human pleasure of breaking off a piece and popping it in your mouth.
Simon Cotton is senior lecturer in chemistry at the University of Birmingham.